As the size of features of integrated circuits decreases, it is increasingly important to reduce the resistance-capacitance delay (RC delay) attributable to interconnects used in such circuits. One approach is to use interconnects having a reduced dielectric constant (k), which can be obtained, for example, by using appropriate low-k materials. In one example, carbonated silicon dioxide (SiOC) films are conventionally known in 90-120 nm technology nodes. A further known approach is to further reduce the dielectric constant by using porous carbonated silicon dioxide films.
The term “carbonated silicon dioxide films” and the corresponding formula “SiOC” are used to designate silicon dioxide films including carbon therein (e.g., by using CH3SiH3 in place of the SiH4 that is often used as a precursor in CVD deposition of a silicon dioxide layer). Such films are sometimes also referred to in the art as carbon-doped silicon dioxide films.
Carbonated silicon dioxide films are being developed by several vendors, using chemical vapor deposition or spin-on coating techniques. Several vendors are currently developing CVD-deposited SiOC films using a “porogen” approach. With this technology, the porogens are built into a dielectric film and are degassed during the post-treatment, leaving pores in the film. Applied Materials (Black Diamond IIx; III), Novellus systems (ELK Coral), Trikon (Orion), and ASM are amongst the companies working on this approach. Suppliers of spin-on porous dielectric materials include Dow Chemicals (SiLK), Rohm & Haas (Zirkon), and JSR.
However, it is known in the art that a silicon oxide-containing material (like a carbonated silicon dioxide) has a substantial population of surface hydroxyl (sometimes referred to herein as “silanol”) groups on its surface. These groups have a strong tendency to take up water because they are highly polarized. They are generated by the breakup of four- and six-member bulk siloxane (Si—O—Si) bridges at the surface of the material. These siloxane structures at the material surface have an uncompensated electric potential and so can be considered to be “strained”. They react readily with ambient moisture to form the surface hydroxyl groups. If the silicon oxide-containing material is porous, the surface hydroxyls and the adsorbed water molecules tend to propagate into the bulk of the material, causing, for reasons known in the art, an increase in the dielectric constant and reduced film reliability.
A comparable effect occurs in materials such as metal oxides because the metal ion-oxide bonds located at the surface of the material have an uncompensated electric potential. This likewise leads to a ready reaction with ambient moisture so as to form surface hydroxyl groups. Once again, if the material is porous, the surface hydroxyls and adsorbed water molecules may propagate into the bulk of the material and lead to an unwanted increase in dielectric constant.
As mentioned above, carbonated silicon oxide is often used as a porous dielectric material. Its carbon-rich surface has relatively fewer strained oxide bonds. Thus, there is a reduced population of surface hydroxyls at the surface of the material at the outset.
However, the tendency for water uptake is still quite high in carbon-containing porous dielectric materials after a dry etch process. The oxidizing plasma reduces the carbon content at the surface of the material and therefore increases the population of surface hydroxyls. The dielectric constant k therefore tends to increase after dry etching, so the k value of the film must be “restored.” A conventional example restoring the dielectric constant uses a supercritical CO2 treatment with hexamethyldisilazane (HMDS).
Besides negatively affecting the dielectric constant of the porous dielectric layer, adsorbed water can also cause problems during subsequent stages in the manufacture of the circuit, notably degassing and reliability problems.
For the reasons described above, it is important to prevent water adsorption and uptake if porous dielectric materials are used to form interconnects. Moreover, moisture uptake in a porous dielectric could possibly corrode metallic barrier layers subsequently formed thereon.
Some known approaches to prevent or impede moisture uptake by porous dielectric materials during manufacture and use of a semiconductor integrated circuit include “dielectric restoration” as mentioned hereinabove, as well as “pore sealing.”
Pore sealing involves prevention of access to the pores in the porous material, for example, by modifying the surface of the porous material (e.g. using an organosilane treatment). Alternatively, a thin dielectric film may be deposited on the surface of the porous dielectric layer. More particularly, the thin dielectric film can be applied to the porous dielectric layer after vias have been etched therein.
Patent Application No. PCT/EP2005/001510 (filed Feb. 15, 2005) describes a technique for cleaning via and trench structures after an etching step, using liquid cleaning agents. Patent Application No. PCT/EP2005/010688 (filed Sep. 1, 2005) describes a composition for passivating a porous, low dielectric constant dielectric layer while simultaneously providing reaction sites promoting the electroless metal layer deposition thereon.